1. Field of the Invention
The present invention relates generally to an information recording medium, and more particularly to a perpendicular magnetic recording medium having a perpendicularly magnetized film.
2. Description of the Related Art
With a reduction in size and an increase in capacity of a magnetic disk drive, it has recently been desired to reduce the grain size of magnetic particles in a recording medium. In a conventional recording method referred to as an in-plane (longitudinal) recording method, it is difficult to greatly reduce the grain size of magnetic particles because of thermal instability. Accordingly, a perpendicular magnetic recording method superior to the in-plane recording method in thermo-magnetic relaxation or the like has recently been investigated. A general perpendicular magnetic recording method employs a dual-layer medium fabricated by forming a soft-magnetic backing layer (base layer) on a substrate and forming a perpendicularly magnetized film on the soft-magnetic backing layer with a nonmagnetic layer interposed therebetween.
In a perpendicular magnetic recording medium for a hard disk, the use of a multilayer film of Co/Pd or Co/Pt as the perpendicularly magnetized film is being investigated. This multilayer film is formed by alternately depositing very thin magnetic films each having a thickness of 0.05 to 2 nm and very thin nonmagnetic films each having a thickness of 0.1 to 5 nm. As compared with a conventional recording medium using Co—Cr based alloy, this multilayer film exhibits much stronger perpendicular magnetic anisotropy, and it is therefore a promising candidate for the perpendicularly magnetized film.
However, the above-mentioned multilayer film has the following problems.
(1) Large Medium Noise Due to a Continuous Film
The medium noise is caused mainly by transition noise or reverse magnetic domain noise accompanying magnetization reversal. The transition noise originates from grain boundary noise by magnetic crystalline particles or nonuniformity of magnetization reversal near the boundary of a recorded magnetic domain. The reverse magnetic domain noise is due to nonuniformity of magnetic particles, demagnetizing fields in a recording film, or leakage magnetic fields from the periphery of a recorded magnetic domain. A general measure against this medium noise is to form a film structure so that adjacent magnetic particles are magnetically isolated from each other. By such magnetic isolation of the adjacent magnetic particles, the medium originated noise (medium noise) can be reduced to improve an S/N and accordingly improve a linear recording density.
The magnetic characteristics of the medium are largely changed by magnetically isolating the adjacent magnetic particles. That is, the coercive force is increased, and the slope α (=4πdM/dH) near the coercive force in an M-H loop is decreased (α=1 in an ideal condition). As a technique for magnetically isolating the adjacent magnetic particles in the Co/Pd or Co/Pt multilayer magnetic film, a method of depositing films by sputtering under high pressures of gases or a method of using a granular base layer is known. However, even though these methods are used, it is not easy to form a single magnetic domain. Further, the control of magnetic characteristics by only one recording film is difficult.
(2) Increase in Noise by the Backing Layer
Since the backing layer itself is formed of a magnetic material, a magnetic flux originating from the magnetization in the backing layer is superimposed on a reproduced signal by a head and accordingly becomes a noise source. The backing layer is a soft-magnetic film having a coercive force (Hc) of several oersteds (Oe), so that it is susceptible to an external magnetic field. Accordingly, a leakage magnetic field from the backing layer is superimposed on a recording magnetic field from a recording head. As a result, uniform perpendicular magnetic recording cannot be attained because of the leakage magnetic field from the backing layer as a noise source. A general measure for reducing the noise due to the backing layer is to form a magnetic domain control layer as an antiferromagnetic film adjacent to the backing layer, thereby controlling magnetic domains in the backing layer. However, the noise cannot be sufficiently reduced by this method.